FOrd did that so it can advertise a higher payload. When You add a bigger fuel tank, that takes away from the payload you can carry. Well, not really, but they are trying to keep the value under 10,000 lbs. If they go over that, they have to classify the truck as something else? Like they do the F350, F450, etc. This is why F350's are higher insurance costs, because insurance companies see that higher weight rating as an opportunity to call the vehicle a commercial vehicle (technically 26,0001 lbs is commercial), and then raise the rates. AT least this is what I was told by an insurance agent.
You are not suppose to exceed over 10,000 pounds with all cargo, fuel, fluids, you, passengers, etc. I think that's right if I remember correctly. So they put smaller tanks in to reduce the weight of the fluids, and to raise the weight that can be carried in the bed/cargo area.
Technically, if you get pulled over by a state patrol officer who is bored that night and can't seem to catch the truckers, he could look at the vehicle weight rating of your door tag, set up portable scales, and if you are over that, could right you a ticket. Unlikely, but he could if he was an A$$hole.
If i put in a new fuel tank, it will also be the larger titan, but I won't do it for the added fuel. I'll do it because I know that ford had some problems with fuel tanks in the 99-07 trucks. The coating inside was failing and getting into the fuel pump and injectors and causing damage. So I won't buy another OEM fuel tank if I replace mine.
